Heat assisted magnetic recording (HAMR) has been proposed as a means by which the recording density of magnetic hard disc drives may be extended to 1 Tb/in2 or higher. Current conventional hard disc drive technology is limited by the superparamagnetic effect, which causes the small magnetic grains needed for high density recording media to gradually lose their magnetization state over time due to thermal fluctuations. By using heat assisted magnetic recording, the magnetic anisotropy of the recording medium, i.e. its resistance to thermal demagnetization, can be greatly increased while still allowing the data to be recorded with standard recording fields. A laser beam heats the area on the disc that is to be recorded and temporarily reduces the anisotropy (and hence coercivity) in just that area sufficiently so that the applied recording field is able to set the magnetic state of that area. After cooling back to the ambient temperature, the anisotropy returns to its high value and stabilizes the magnetic state of the recorded mark.
HAMR requires an apparatus that is able to conduct sufficient light energy into the recording medium to heat it by several hundred degrees, but only in the area that is desired to be recorded, which typically will have dimensions on the order of 25 to 50 nm if the recording density is 1 Tb/in2. If the optical hot spot is larger than this area, it will extend to neighboring bits and tracks on the disc, and by heating those areas as well, will cause the data recorded in those areas to be erased eventually. Confining the optical spot to an area that is much smaller than a wavelength of light, and well below the so-called “diffraction limit” that can be achieved by standard focusing lenses, is an area of study called “near field optics” or “near field microscopy.”
Solid immersion mirrors have been proposed for use in optical transducers in HAMR recording systems. In some applications envisioned for the use of a solid immersion mirror, a device (such as a metal pin) can be placed at the focus of the mirror, which is often in the shape of a parabola, in order to achieve an additional function, for example, such as producing an optical spot well below the diffraction limit of the mirror. However, in order to be used in this manner, the solid immersion mirror would have to be terminated. Light entering a terminated solid immersion mirror near the central axis is not reflected toward the focal point. The lost rays in the central portion can be significant since it is likely that the intensity profile of the electromagnetic wave across the solid immersion mirror will not be uniform but rather Gaussian (or similar) so that the most intense light (near the central axis) would be lost due to the termination of the solid immersion mirror.
This invention provides a transducer design that provides improved transfer of optical energy to a focal point.